Data transmitting and receiving system, terminal, relay device, and data transmitting method

ABSTRACT

The terminal includes: the transmitting and receiving unit ( 908 ) transmitting and receiving the observation packet for observing the transmission quality and data; the transmission quality managing unit ( 906 ) (i) exchanging the observation packet with another terminal via the transmitting and receiving unit ( 908 ), (ii) observing a loss rate of the data between the terminal and the other terminal and writing to the observation packet the degraded state information indicating the loss rate in the case where the terminal receives the data, and (iii) subtracting the congested state information from the degraded state information included in the observation packet collected from the other terminal in order to calculate a transmission error rate of the data in the terminal and the relay device in the case where the terminal transmits the data; and the error correction code processing unit ( 905 ) determining a forward error correction capability based on the transmission error rate calculated by the transmission quality managing unit ( 906 ) and assigning to the data to be transmitted the forward error correction code according to the forward error correction capability.

TECHNICAL FIELD

The present invention relates to a data transmitting and receivingsystem performing transmission and reception of data between terminalsconnected to a network via a relay device.

BACKGROUND ART

There have been proposed techniques to realize packet communicationsreducing delay in an environment of a network (typically referred to asan ad-hoc network, or a mesh-type network) which entirely consists ofwirelessly connectable terminals (personal computer), the PersonalDigital Assistant (PDA), cellular phone, and car navigation system), anddispenses with an access point such as a wireless Local Area Network(LAN). One of such techniques, for example, is a route switchingtechnique which sets routes between transmitting and receiving terminalsand periodically provides a control packet to each of the routes inorder to measure an end-to-end delay, select one of the routes alwayshaving the least delay, and transmit a packet.

This ad-hoc network, however, has a long transmission delay problem.Besides, the ad-hoc network suffers another problem of data loses causedby the buffer overflow at a relay device interconnecting the terminalsand the resulting congestion, and by occurrence of a transmission errorsince interconnections are established between relaying devices, and arelay device and a transmitting and receiving terminals.

Hence, occurrence of the data losses caused by the congestion and thetransmission error leads to problems of video and audio data qualitydegradation including a distorted image and skipping sound whentransmitting the video and the audio data.

Specifically, determination is to be made in that which cause, thecongestion or the transmission error both developing the data losseswhen conducting wireless transmission, influences the data loss to whatdegree.

In order to overcome the problems, a possible technique is to apply i) atechnique (Patent Reference 2) for setting a transmission rate andintensity of an error correction code, distinguishing between thetransmission error and the congestion, by periodically transmitting anobservation packet to ii) a technique (Patent Reference 1) for correctlymeasuring a traffic amount at the routing device by the periodicaltransmission of an observation packet for observing transmission qualitybetween the transmitting and receiving terminals in the ad-hoc network.

Patent Reference 1: Japanese Unexamined Patent Application PublicationNo. 2005-347879

Patent Reference 2: Japanese Unexamined Patent Application PublicationNo. 2003-152752

DISCLOSURE OF INVENTION

Problems that Invention is to Solve

In Patent Reference 1, the observation packet for observing transmissionquality is relayed typically among plural stages of relay devices.Hence, the increasing number of the stages of the relay devices relayingthe observation packet leads to a longer packet length of theobservation packet, developing overheads such as an analysis of a packetdetail and a communication volume.

In addition, a system involving a multistage relay and dynamicdetermination of a transmission path, including a wireless ad-hocnetwork, faces problems such that assuming a congested relay device ischallenging and specifying the congested relay device is difficult.

The present invention is conceived in view of the above problems and hasas an objective to provide data transmitting and receiving system, in anad-hoc network, to effectively transmit an observation packet, to beable to specify a congested relay device, and to realize high qualitytransmission of data.

Means to Solve the Problems

In order to achieve the above objective, a data transmitting andreceiving system in the present invention includes: a relay device whichis connected to a network; and a first terminal and a second terminaleach of which transmits and receives data via the relay device, whereinthe relay device includes: a receiving unit which receives anobservation packet used for observing transmission quality, and data; areception buffer in which the data is stored; a reception buffermanaging unit which observes a loss rate of the data in the receptionbuffer; a packet information rewriting unit which writes the loss rateinto the observation packet as congested state information, the lossrate being observed by the reception buffer managing unit; atransmission buffer which stores on a destination basis the data storedin the reception buffer; a transmission buffer managing unit whichforwards the data stored in the transmission buffer at a predeterminedtransmission rate on the destination basis; and a transmitting unitwhich transmits towards a destination the data forwarded by thetransmission buffer managing unit, and the observation packet, a firstterminal includes: a first transmitting and receiving unit whichtransmits and receives data and the observation packet; and a firsttransmission quality managing unit which writes degraded stateinformation in the observation packet, the degraded state informationindicating the loss rate of the data between the first terminal and thesecond terminal, and the second terminal includes: a second transmittingand receiving unit which transmits and receives data and the observationpacket; a second transmission quality managing unit which calculates atransmission error rate of the data in the data transmitting andreceiving system by subtracting the loss rate indicated in the congestedstate information from the loss rate indicated in the degraded state,the loss rates being included in the observation packet obtained fromthe first terminal; and an error correction processing unit whichexecutes processing on data to be transmitted, so that an error of thedata is corrected when the data is received, the processing beingexecuted according to the transmission error rate calculated by thesecond transmission quality managing unit.

According this structure, the observation packet is transmitted from atransmitting terminal (the second terminal) toward a receiving terminal(the first terminal) via the relay device.

Upon receiving the observation packet, the relay device writes into theobservation packet congested state information indicating a data lossrate at the relay device, and relays the observation packet. Further,the receiving terminal writes into the observation packet the degradedstate information indicating the data loss rate between the twoterminals, and returns the observation packet to the transmittingterminal. Thus, the transmitting terminal can specify a congested relaydevice, using the congested state information written into theobservation packet. In addition, subtracting the congested stateinformation from the degraded state information makes possiblecalculating a transmission error in the data transmitting and receivingsystem. By providing processing to data to be transmitted for correctingan error of the data in the transmitting according to the transmissionerror rate, a high-quality transmission can be realized. Further, alength of the observation packet is independent from the number ofstages to be relayed. Thus, efficient transmission of the observationpacket is also possible.

Preferably, the first terminal further includes a first terminalreception state managing unit which detects a change of a receptionstate of the data in the transmitting and receiving unit, and, in thecase where the change of the reception state is determined, to instructthe transmission quality managing unit to transmit the observationpacket, the second terminal further includes a second terminal receptionstate managing unit which detects a change of a reception state of thedata in the transmitting and receiving unit, and, in the case where thechange of the reception state is determined, to instruct thetransmission quality managing unit to transmit the observation packet,the first transmission quality managing unit transmits, via the firsttransmitting and receiving unit, the observation packet towards aterminal of a destination of the data in response to the instructionfrom the first terminal reception state managing unit so as to transmitthe observation packet, and the second transmission quality managingunit transmits, via the second transmitting and receiving unit, theobservation packet towards a terminal of a destination of the data inresponse to the instruction from the second terminal reception statemanaging unit so as to transmit the observation packet.

In addition, the relay device is further characterized to include arelay device reception state managing unit which detects a change of areception state of the data in the receiving unit, and, in the casewhere the change of the reception state is determined, to instruct thesecond transmission quality managing unit so as to transmit theobservation packet, and the second transmission quality managing unit ischaracterized to transmit, via the second transmitting and receivingunit, the observation packet towards the first terminal in response tothe instruction from the relay device reception state managing unit soas to transmit the observation packet.

This structure allows the transmitting terminal to transmit theobservation packet when a change occurs in a reception state of thedata. This allows the transmitting terminal to avoid transmittingunnecessary observation packets, so that loads delivered to the network,the terminals and the relay device can be reduced.

Further preferably, in the case where the receiving unit receives withina predetermined time period a plurality of instructions including theinstruction for transmitting the observation packet from a same sourceto a same destination, the relay device reception state managing unit ischaracterized to execute one of the instructions on the secondtransmission quality managing unit, and to discard remaininginstructions out of the instructions.

When receiving plural instructions for transmitting an observationpacket to be transmitted from the same source to the same destination ina short period of time, the relay device keeps one of the triggerpackets and discards the rest. This can prevent two or more triggerpackets having instructions with the same details from being transmittedtowards the transmitting terminal. This can alleviate loads delivered tothe network, the terminal, and the relay device.

It is noted that the present invention can be implemented in a form of adata transmitting method realizing characteristic units in the datatransmitting and receiving system as steps, and a program causing acomputer to execute a characteristic steps included in the datatransmitting method, in addition to a form of the data transmitting andreceiving system including the above characteristic units. As a matterof course, such a program can be distributed via a storage mediaincluding a Compact Disc-Read Only Memory (CD-ROM), and a communicationsnetwork including the Internet.

Effects of Invention

In a wireless ad-hoc network, a transmitting apparatus can specify acongested relay device, using congested state information written in anobservation packet. Further, the transmitting apparatus can obtain atransmission error rate, using degraded state information and thecongested state information. By providing processing to data to betransmitted for correcting an error of the data in the transmittingaccording to the transmission error rate, a high-quality transmissioncan be realized.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a structure of a network intended in the presentinvention.

FIG. 2 illustrates a forward error correction code used by atransmitting and receiving terminal.

FIG. 3 illustrates an outline of a transmission technique distinguishingbetween congestion and a transmission error in the present invention.

FIG. 4 illustrates a packet structure of an observation packet.

FIG. 5 illustrates a packet structure of a trigger packet.

FIG. 6 illustrates a structure of a terminal.

FIG. 7 illustrates a structure of a relay device.

FIG. 8 illustrates an operation of a reception state managing unit.

FIG. 9 is a flowchart showing an operation of an entire system.

FIG. 10 is a flowchart showing an operation of the relay device.

FIG. 11 is a flowchart showing transmission processing of theobservation packet.

FIG. 12A illustrates a scene showing a specific usage of a wirelessad-hoc network.

FIG. 12B illustrates a scene showing a specific usage of a wirelessad-hoc network.

FIG. 13 illustrates a control technique of the observation packet andthe trigger packet based on a usage scene.

NUMERICAL REFERENCES

-   901 Displaying unit-   902 Decoding unit-   903 Input unit-   904 Transmission rate controlling unit-   905 Error correction code processing unit-   906 Transmission quality managing unit-   907 and 1005 Reception state managing unit-   908 Transmitting and receiving unit-   1001 Receiving unit-   1002 Reception buffer managing unit-   1003 Packet information rewriting unit-   1004 Destination distributing unit-   1006 Routing processing unit-   1007 Transmission buffer managing unit-   1008 Transmitting unit-   1010 Trigger packet image-   1020 Reception state managing table

BEST MODE FOR CARRYING OUT THE INVENTION

Described hereinafter is an embodiment of the present invention withreference to the drawings.

FIG. 1 illustrates a structure of a network intended in the presentinvention.

As shown in FIG. 1, terminals are connected to a relay device. Each ofrelay devices forms an ad-hoc network on a wireless network, and isinterconnected with a terminal on a wire network.

A relay device interconnecting the wireless network and the wire networkis also referred to as a gateway. Plural gateways or no gateways may beutilized.

Each of the relay devices, having a routing function, establishesinterconnections between the relay apparatuses in the wireless ad-hocnetwork, and the relay device and the wire network. Here, the routingfunction means selecting the most appropriate route from a transmittingterminal to a receiving terminal in order to transmit data via the relaydevice.

The routing function may be achieved in an Internet

Protocol (IP) packet level, or in a datalink layer, such as theInstitute of Electrical and Electronic Engineers (IEEE) 802.11s. Therouting function may be implemented on a routing protocol. Typicalprotocols are the Dynamic Source Routing (DSR) Protocol and the Ad hocon-demand Distance Vector (AODV). In general, the routing protocolgenerates path information.

Each of the terminals shown in FIG. 1 includes a camera and a PersonalComputer (PC). Here, no designated kind or performance of the terminal,such as a cell phone, a TV, and a car navigation system, may be defined.Further, the terminal may be mobile or stationary.

The wireless network includes a wireless LAN, the Dedicated Short RangeCommunication (DSRC), and a cellular phone network. Thus no designatedkind or performance is defined for the wireless media. Similarly, thewire network includes a fiber optical network, the Public SwitchedTelephone Networks (PSTN), and the LAN. Thus no designated kind orperformance is defined for the wire network media.

The network shown in FIG. 1 is applicable to transmitting various media,such as text, a still picture, music, and control data, as well astransmitting a video and a speech. Further, the network in FIG. 1 issubject to no limited usage locations, and covers an in-househigh-quality medium as well as an outside medium for transmission.

FIG. 2 illustrates a forward error correction code used by atransmitting and receiving terminal.

Retransmission of a transmission packet and utilization of the ForwardError Correction (FEC) has been adopted in order to normally recover atransmission packet developing a transmission packet loss and a biterror due to congestion and a transmission error.

The FEC is a technique to add redundant information to informationintended to be transmitted in order to correct, based on the redundantinformation, an error of information developed during the transmission.The error correction capability improves as an error correction codebecomes longer. After the correction, however, the information havingthe longer error correction code is slower in transmission speed thaninformation with no error correction when transmitting at the samespeed.

The most suitable length of an error correction code depends onreliability of a communication path; meanwhile, correction capabilitymay vary, among error correction codes having the same length, accordingto an algorithm (difference in format). Exemplified as error correctioncode techniques are the Reed-Solomon code, a block code represented bythe Bose-Chaudhuri-Hocquenghem (BCH) code, and a convolutional coderepresented by the Vitrbi code. Another example of the error correctioncode techniques is described in Patent Reference: Japanese UnexaminedPatent Application Publication No. 2001-115238 with reference to H. 223(the standard which defines techniques for multiplexing moving picturedata, audio data, and communications data, in videophones, recommendedby the ITU). The introductory part of the background art in PatentReference emphasizes on the necessity of a technique selecting a levelof error resilience depending on a condition of a transmission path.

Specifically described next is a technique for utilization of the FEC inthe present invention.

A protocol layer which the FEC is provided includes the applicationlayer and the data link layer.

FIG. 2 exemplifies the FEC to be provided to entire coded AV data in theapplication layer. Each of nodes (a device indicating the use of atleast the IP layer and the MAC layer), corresponding to one relaydevice, shows a network protocol layer to be used.

Internet Drafts (a series of working documents in the Internet), forexample, define a technique for utilization of an error correction codein the application layer with reference to Non-Patent Reference:Reed-Solomon Forward Error Correction (FEC),draft-ietf-rmt-bb-fec-rs-01.txt. Internet Drafts also define the factthat selection for correction capability is available.

A missing block due to the congestion is recovered by the receivingterminal with a use of the FEC (referred to as FEC in the APP(Application) layer in FIG. 2) provided in the application layer.

Moreover, the data link layer involves dividing data from theapplication layer into blocks, and providing the FEC (referred to as FECin the Media Access Control (MAC) layer in FIG. 2) to each of the blocksin order to give resilience against the bit error occurred during thetransmission.

For example, the IEEE802.11a defines the convolutional coding, theVitabi coding, and a use of intra-symbol interleave. The convolutionalcode has a constraint length of 7, and is defined to be selected out of½, ⅔, and ¾ in code rate.

The FEC in the application layer is used to recover a packet lossdeveloped due, to the congestion and the transmission error between thetransmitting and receiving terminals. Meanwhile, the FEC in the datalink layer is used to recover a packet loss developed due to thetransmission error between relay devices.

Further, selection of presence or absence in setting, a technique, andintensity of the error correction code is available in each of layers;however, no definition is provided for selecting the technique and theintensity specifically in what timing and how.

FIG. 3 illustrates an outline of a transmission technique in the presentinvention, distinguishing between congestion and a transmission error.

Transmitted and received between the transmitting and receivingterminals (a transmitting terminal and a receiving terminal) is anobservation packet for measuring: degraded state information (a packetloss rate and a delay time between the transmitting and receivingterminals) of transmission quality; and congested state information (apacket loss rate and a delay time at each of relay devices) of the relaydevice. Each of the relay devices, measuring the congestion conditioninformation, writes the congested state information into the receivedobservation packet. The receiving terminal measures a degraded state(the packet loss rate and a transmission delay time between thetransmitting and the receiving terminals) of the transmission qualitybetween the transmitting and the receiving terminals, writes themeasured degraded state information into the received observationpacket, and returns the observation packet to the transmitting terminal.Based on the observation packet, the transmitting terminal distinguishesbetween the transmission error rate and the congestion condition, anduses each of the distinguished transmission error rate and thecongestion condition to control the forward error correction codetechnique or the intensity, and to control the transmission rate,respectively.

In addition, based on a change of a reception state of the transmittingand receiving terminal and the relay device, and a change of a routingrenewal condition of the relay device, the receiving terminal or therelay device transmits a trigger packet towards the transmittingterminal. This allows the transmitting terminal to comprehend the changeof a reception state in the receiving terminal and routing renewal ofthe relay device which cannot be detected at the transmitting terminal.The transmitting terminal receives the trigger packet. In response tothe reception, the transmitting terminal transmits the observationpacket. This eliminates the need for transmitting an unnecessaryobservation packet, and makes possible measuring the degraded stateinformation of the transmission quality on the transmission path and thecongested state information of the relay device. It is noted that thedetection of the reception state change may be achieved by detecting:

reception field intensity, notification information of handover, atransmission speed, a selected forward error correction technique orselected intensity, a transmission error rate, or a change ofinformation on the number of retransmission.

(Change of Reception State)

A change of reception state is observed at each of the transmittingterminal, the receiving terminal, and the relay device. When thetransmitting and receiving terminal detects handover (a switch betweenrelay devices for relaying), a connection condition has been changed (adestination relay device to which the terminal has been connected isswitched to another destination relay device) between the transmittingterminal and receiving terminal. The change of the reception state atthe receiving terminal, as described above, requires transmission andreception of the observation packet between the transmitting andreceiving terminals. Typical handover processing (a switching processbetween transmission paths, following the switch of the destinationrelay devices) is triggered when reception field intensity of the relaydevice or the terminal decreases, or the transmission error rateincreases based on a predetermined threshold. Even though no connectioncondition change is observed, an available bandwidth has been changedwhen the transmitting and receiving terminal detects a decrease intransmission speed to the predetermined threshold due to a significantchange of a transmission environment, such as an appearance of anobstacle. Thus, the above case requires transmission of the observationpacket, as well. In general, degradation of transmission quality causesa change in transmission speed. Hence, the change of information in theforward error correction technique or the intensity, the transmissionerror rate, or the number of retransmission may be detected, as well asthe transmission speed. It is noted that the change of the receptionstate may be detected by using either each pieces of the aboveinformation alone or a combination of the pieces of the information.

FIG. 4 illustrates a packet structure of the observation packet.

As specifications of the observation packet, in the present invention,maximizing the performance thereof, the observation packet includesdescription of information between the transmitting and receivingterminals and description of information on a relay node, as well as theIP header and the UDP (User Datagram Protocol) header. Described in thedescription of information between the transmitting and receivingterminals are a “transmission time” at which the transmitting terminaltransmits the observation packet, a “loss rate” indicating a packet lossper unit time when data is transmitted from the transmitting terminal tothe receiving terminal, and a “delay time” required for the datatransmission from the transmitting terminal to the receiving terminal.It is noted that a serial number for determining a transmission orderand missing data, and the transmission time used for measuring the delaytime of each packets can be described, in the data to be transmitted,per packet for data transmission. An average value of delay times ofassociated packets is described in the “delay time” in the descriptionof information between the transmitting and receiving terminals.

Further, described in the description of information on the relay nodeare a “relay device ID” identifying a device at which the data isrelayed, a “measurement time” of a congested state in the relay device,a “loss rate” (a packet loss per unit time at the relay device)indicating the congested state in the relay device, and a “stay time”(an average time between reception and transmission of data at the relaydevice).

It is noted in Patent Reference 1 that each of the relay devices writesan additionally observed congested state to the above description ofinformation on the relay node for every relay device the data passesthrough. In order to curb the overhead, including the packet analysisand the communication volume at the relay device, which Patent Reference1 faces as a problem, the present invention may use the description ofinformation between the transmitting terminals alone, instead of thedescription of information on the relay node, in order to obtain thetotal sum of congested states of all the relay devices.

Specifically, in transmitting the observation packet from thetransmitting terminal, the transmission time in the informationdescription between the transmitting and receiving terminals is set to atransmission time of the transmitting terminal, and the loss rate andthe delay time are set to zero. Each relay device reads items of theloss rate and the delay time in the information description betweentransmitting and receiving terminals, adds a packet discarding rate at abuffer of the relay device and an average stay time of the packet in thebuffer of the relay device to a value of the item of the loss rate and avalue of the item of the delay time, respectively, and overwrites totransmit the added values to the associated items indicating the lossrate and the delay time in the information description between thetransmitting and receiving terminals. It is noted that the observationpacket is preferably processed separately from a data transmissionpacket in the relay device in order to keep data loss due to congestionfrom occurring.

The receiving terminal reads the observation packet which has beenrelayed via the relay devices to obtain, out of items indicating theloss rate and the delay time in the information description between thetransmitting and receiving terminals, a total sum of the packet lossrates (L1) and a total sum of the delay times (T1) developed out ofcongestion at all the relay devices. Out of a difference between thereception time of the observation packet and the transmission time ofthe information description between the transmitting and receivingterminals, the receiving terminal calculates a total delay time (T2),between the transmitting and receiving terminals, regarding the delaytime developed out of the congestion and the transmission error betweenthe transmitting and receiving terminals. Out of a difference between T2and T1, the receiving terminal calculates a total delay time (T3) mainlydeveloped by the transmission error, rather than by the congestion.

Further, each of data transmission packets, transmitted separately fromthe observation packet, intends to have a sequential serial numberassigned. Some of the data packets transmitted from the transmittingterminal are discarded when congestion or a transmission error occurs inrelaying by the relay device. The receiving terminal measures a missingratio of the serial number assigned to the data packet to calculate atotal loss rate (L2) regarding the packet discarding occurred by thecongestion and the transmission error between the transmitting andreceiving terminals. Calculating the difference between the L2 and L1makes possible obtaining a total loss rate (L3) developed by thetransmission error.

The receiving terminal transmits the received observation packet to thetransmitting terminal, as well as calculates L3 and T3 out of L1 and T1,respectively. Based on a value of L1 or T1 of the observation packetwhich the transmitting terminal receives from the receiving terminal,the transmitting terminal adjusts a transmission amount therefrom.Specifically, the transmitting terminal determines the development ofthe congestion and reduces the transmission amount when the value of L1or T1 is greater than a predetermined threshold value (Th1), anddetermines the reduction of the congestion and increases thetransmission amount when the value of L1 or T1 is smaller than apredetermined threshold (Th2). It is noted that Th1 and Th2 may beempirically determined.

Further, the value of L3 or T3; that is the transmission qualityinformation on the transmission error calculated by the receivingterminal, is either additionally written in the observation packet, ortransmitted to the transmitting terminal as a separate packet. However,the information on the transmission error does not necessarily requiretransmission to the transmitting terminal. In transmitting, theinformation on the transmission error does not need to be transmitted atthe same timing as the observation packet is transmitted.

Based on the values, the transmitting terminal determines that thenumber of transmission errors is increasing in the case where the valueof L3 or T3 is greater than the predetermined threshold (Th3) andenhances a transmission error capability. Meanwhile, the transmittingterminal determines that the number of transmission errors is decreasingin the case where the value of L3 or T3 is smaller than thepredetermined threshold (Th4) and decreases the transmission errorcapability. When the transmission error capability is intended to be setin the application layer, as shown in FIG. 5, a forward error correctiontechnique and a redundancy degree for data to be transmitted areselected. When the transmission error capability is intended to be setin the data link layer, the forward error correction technique and theredundancy degree for a targeted wireless transmission path areselected. It is noted that Th3 and Th4 may be empirically determined.

FIG. 5 illustrates a packet structure of the trigger packet.

The trigger packet includes an “event occurrence time”, an “occurrencelocation”, and an “occurrence cause” in addition to the

UDP header and the IP header. The “event occurrence time” representstimes indicating: a change of a reception state at the transmittingterminal and the relay device; and a switch of routing paths at therelay device. The “occurrence location” is information (an IP address,for example) specifying locations (transmitting terminal or relaydevice) at which such a change and a switch occur. Described in the“occurrence cause” are a reception field intensity, a transmissionspeed, handover, and routing as specific details of the change of thereception state at the transmitting terminal and the relay device.

FIG. 6 illustrates a structure of the terminal.

The terminal; that is a transmitting terminal transmitting data and areceiving terminal receiving data, includes a displaying unit 901, adecoding unit 902, an input unit 903, a transmission rate control unit904, an error correction code processing unit 905, a transmissionquality managing unit 906, a reception state managing unit 907, and atransmitting and receiving unit 908.

The displaying unit 901 displays decoded data.

The decoding unit 902 decodes data which requires decoding, such asvideos and speeches.

The input unit 903 is capable of inputting different kinds of media dataincluding videos, still pictures, speeches, music, text, computergraphics (CG), and control information.

The transmission rate control unit 904 determines the transmission rateout of the loss rate and the delay time caused by the congestion.

The transmission quality managing unit 906 distinguishes between a losscaused by the congestion and a loss caused by the transmission error andextracts the losses according to the observation packet.

The reception state managing unit 907 detects the change of thereception state of the transmitting and receiving terminal.

The error correction code processing unit 905 determines presence orabsence, a scheme, and an intensity of the forward error correction codein the application layer according to the transmission error rate.

The transmitting and receiving unit 908 transmits and receives data withthe forward error correction code assigned.

To be more specific, the terminal includes: the transmitting andreceiving unit 908 transmitting and receiving the observation packet forobserving the transmission quality and data; the transmission qualitymanaging unit 906 (i) exchanging the observation packet with anotherterminal via the transmitting and receiving unit 908, (ii) observing aloss rate of the data between the terminal and the other terminal andwriting to the observation packet the degraded state informationindicating the loss rate in the case where the terminal receives thedata, and (iii) subtracting the congested state information from thedegraded state information included in the observation packet collectedfrom the other terminal in order to calculate a transmission error rateof the data in the terminal and the relay device in the case where theterminal transmits the data; the reception state managing unit 907detecting the change of the reception state of the data at thetransmitting and receiving unit 908 and, in the case where the receptionstate managing unit 907 determines that the reception state has changed,directing the transmission quality managing unit 906 to transmit theobservation packet; and the error correction code processing unit 905determining a forward error correction capability based on thetransmission error rate calculated by the transmission quality managingunit 906 and assigning to the data to be transmitted the forward errorcorrection code according to the forward error correction capability.

It is noted in the transmitting terminal that when the reception statemanaging unit 907 detects the change of the reception state at receptionstate managing unit 907, and when the transmission quality managing unit906 receives the trigger packet, the transmission quality managing unit906 in the transmitting terminal transmits the observation packet to thereceiving terminal. Further, the transmitting and receiving unit 908transmits the data, the reception state information, and the observationpacket and the trigger packet to the decoding unit 902, the receptionstate managing unit 907, and the transmission quality managing unit 906,respectively.

It is noted that the detection of the change of the reception state maybe achieved by detecting the reception field intensity, the notificationinformation of handover, the transmission speed, the selected forwarderror correction technique or the selected intensity, the transmissionerror rate, or the change of information on the number ofretransmission.

Specifically, the reception state managing unit 907 detects the changeof the reception state at the transmitting and receiving unit 908. Forexample, the reception state managing unit 907 determines that thereception state has changed in the case where (i) the reception fieldintensity goes below a predetermined threshold for a certain period oftime, (ii) the relay device detects the fact that the receiving terminalhas moved to belong to another area, and then transmits to the receivingterminal the notification information of handover, (iii) thetransmission speed or a transmission mode changes and the forward errorcorrection technique or the intensity is changed since the transmissionerror rate goes higher than a predetermined threshold for a certainperiod of time, and (iv) the number of retransmission goes higher than apredetermined threshold due to a packet loss caused by the congestionand the transmission error. On the contrary, when transmission conditionimproves with respect to a threshold, the reception state managing unit907 determines that the reception state has changed. In the case wherethe terminal is a transmitting terminal, the reception state managingunit 907 prompts the transmission quality managing unit 906 to transmitthe observation packet to the receiving terminal only when the receptionstate has changed. In addition, when the terminal is a receivingterminal, the reception state managing unit 907 prompts the transmissionquality managing unit 906 to transmit the trigger packet to thetransmitting terminal. It is noted that the reception state managingunit 907 may determine that the reception state has changed whendetecting that the congested state information included in theobservation packet or the loss rate indicated in the degraded stateinformation has gone higher or lower with respect to a predeterminedthreshold.

FIG. 7 illustrates a structure of the relay device.

The relay device, connected to the terminal via a network and relayingthe data to be transmitted and received between the terminals, includesa receiving unit 1001, a reception buffer managing unit 1002, a packetinformation rewriting unit 1003, a destination distributing unit 1004, areception state managing unit 1005, a routing processing unit 1006, atransmission buffer managing unit 1007, and a transmitting unit 1008.

The receiving unit 1001 receives the data transmitted from another relaydevice and the terminal.

The reception buffer managing unit 1002 temporality accumulates thereceived data and, when a reception buffer is about to overflow,discards a packet according to a predetermined rule (for example,discarding transmission packets at random when the reception buffer isoccupied up to ⅔ of the capacity (threshold)).

The packet information rewriting unit 1003 collects the congested stateinformation (loss rate, stay time, and measurement time) of thereception buffer and, when the observation packet is found in thereception buffer, writes the congested state information into theobservation packet. It is noted that the packet information rewritingunit 1003 does not perform processing on any other packet than theobservation packet.

The destination distributing unit 1004 determines the next relay deviceas a transmission destination of a packet stored in the reception buffermanaging unit 1002 according to a packet destination table (routingtable) generated by the routing processing unit 1006.

The reception state managing unit 1005 detects a change of a receptionstate (including reception field intensity and handover) at thereceiving unit 1001 in the relay device.

The routing processing unit 1006 extracts to analyze a routing protocolfrom the receiving unit 1001, and generates the packet destinationtable.

The transmission buffer managing unit 1007 prepares a transmission queuefor each of destinations, and transmits a predetermined transmissionamount of packets at a predetermined timing.

The transmitting unit 1008 transmits the data.

To be more specific, the relay device, connected to the terminal via anetwork and relaying the data to be transmitted and received between theterminals, includes: the receiving unit 1001 receiving the data and theobservation packet for observing the transmission quality; the receptionbuffer managing unit 1002 (i) having a reception buffer for storingdata, (ii) storing the received data in the reception buffer, (iii)discarding data failed to be stored in the reception buffer, and (iv)observing a data loss rate in the reception buffer; the packetinformation rewriting unit 1003 writing the loss rate observed at thereception buffer managing unit 1002 to the observation packet ascongested state information; the transmission buffer managing unit 1007having a transmission buffer for storing the data stored in thereception buffer on a destination basis, and transmitting the datastored in the transmission buffer at a predetermined transmission rateon a destination basis; the transmitting unit 1008 transmitting thedata, forwarded by the transmission buffer managing unit 1007, towardsthe destination; the reception state managing unit 1005 detecting thechange of the reception state of the data at the transmitting unit 1008,and, in the case of determining the change of the reception state,directing transmission of the observation packet to the transmissionquality managing unit 906 included in the source terminal of the data;the routing processing unit 1006 using a predetermined routing protocolto generate a routing table indicating a transmission path of the data;

and the destination distributing unit 1004 determining the nextdestination of the data according to the routing table.

Similar to the terminal, the reception state managing unit 1005 detectsthe change of the reception state at the relay device. Specifically, thereception state managing unit 1005 detects the reception fieldintensity, the notification information of handover, the transmissionspeed, the selected forward error correction technique or the selectedintensity, the transmission error rate, or the change of information onthe number of retransmission, as well as renewal of the routing table.Detecting the change of the reception state, the reception statemanaging unit 1005 prompts the destination distributing unit 1004 totransmit the trigger packet. It is noted that the reception statemanaging unit 1005 may determine the change of the reception state whendetecting that the loss rate indicated in the congested stateinformation included in the observation packet goes higher or lower withrespect to a predetermined threshold.

For example, the reception state managing unit 907 determines that thereception state has changed in the case where (i) the reception fieldintensity goes below a predetermined threshold for a certain period oftime, (ii) the relay device detects the fact that the receiving terminalhas moved to belong to another area, (iii) the transmission speed or atransmission mode decreases and the forward error correction techniqueor the intensity is changed since the transmission error rate goeshigher than a predetermined threshold for a certain period of time, (iv)the number of retransmission goes higher than a predetermined thresholddue to a packet loss caused by the congestion and the transmissionerror, and (v) the routing is re-established. On the contrary, whentransmission condition improves with respect to a threshold, thereception state managing unit 907 determines that the reception statehas changed.

(Aggregation of Trigger packets at the Relay Device)

In addition, the reception state managing unit 1005 receives triggerpackets from the receiving unit 1001. The reception state managing unit1005: compares a received trigger packet with a previously receivedtrigger packet having the same source and destination as the triggerpacket has; and determines whether or not a time difference between acurrent time (reception time at the reception state managing unit 1005)of the currently received trigger packet and a transmission time (to thedestination distributing unit 1004) of the previously received triggerpacket is within a predetermined time. Based on the determinationresult, the reception state managing unit 1005 determines whether thetrigger packet transmitted from the receiving unit 1001 is to betransmitted or discarded, and executes the resulting determination inorder to make possible reducing the number of the trigger packets, whichleads to reduction of a traffic amount of the packets and the processingamount of the packets at the transmitting and receiving terminal and therelay device.

Specifically, when receiving the trigger packet from the receivingterminal and another relay device, the receiving unit 1001 transmits thereceived trigger packet to the reception state managing unit 1005. Thereception state managing unit 1005 compares data included in thereceived trigger packet with data included in the previously receivedtrigger packet in order to confirm whether or not the trigger packetincluding the same source and destination has already been transmittedoutside within the predetermined time. In the case where the triggerpacket has been transmitted towards the same source and the samedestination, the reception state managing unit 1005 discards a newlyreceived trigger packet, instead of transmitting the newly receivedtrigger packet to the destination distributing unit 1004. On thecontrary, in the case where a time goes after the predetermined time,and the destination of the trigger packet is different even transmittedwithin the predetermined time, the reception state managing unit 1005transmits the newly received trigger packet to the destinationdistributing unit 1004.

FIG. 8 illustrates an operation of the reception state managing unit1005.

FIG. 8 shows a trigger packet image 1010 received by the receiving unit1001. The reception state managing unit 1005 obtains the source addressand the destination address from the IP header of the trigger packet tomeasure a current time indicating when the reception state managing unit1005 receives the trigger packet. The reception state managing unit 1005has not previously received a trigger packet having the same destinationand source as a trigger packet with the packet number 1 has. Thus, thereception state managing unit 1005 transmits the trigger packet to thedestination distributing unit 1004. In addition, the reception statemanaging unit 1005 has not relayed the trigger packet to the destinationaddress “X” shown in the trigger packet with the packet number 1. Thus,the reception state managing unit 1005 records details of the triggerpacket with the packet number 1 in a managing table 1020 in thereception state.

Not having previously performed transmission to the same destination asa trigger packet with the packet number 2 has been transmitted, similarto the trigger packet with the packet number 1, the reception statemanaging unit 1005 transmits the trigger packet with the packet number 2and additionally writes details of the trigger packet with the packetnumber 2 on the managing table 1020 in the reception state.

A trigger packet with the packet number 3 has the same destination andsource addresses as the trigger packet with the packet number 1 has.Further, provided that the predetermined time is assumed to be threeminutes, a time interval between the time (10:01) receiving the triggerpacket with the packet number 1 and the time (10:02) receiving thetrigger packet with the packet number 3 is less than three minutes.Hence, the trigger packet with the packet number 3 is not transmittedand discarded.

A trigger packet with the packet number 4 has the same destination andsource addresses as the trigger packet with the packet number 2 has.However, a time interval between the time (10:02) receiving the triggerpacket with the packet number 2 and the time (10:07) receiving thetrigger packet with the packet number 4 is three minutes or longer.Thus, the trigger packet with the packet number 4 is transmitted, andthe transmission time of the managing table 1020 in the reception stateis renewed. It is noted that when the routing processing unit 1006informs the reception state managing unit 1005 of the renewal of therouting table, the reception state managing unit 1005 transmits thetrigger packet to the destination address described in the managingtable 1020 in the reception state.

Described above is the operation of the reception state managing unit1005 in the relay device; meanwhile, the reception state managing unit907 in the terminal carries out a similar operation.

FIG. 9 is a flowchart showing an operation of an entire system.

The transmission quality managing unit 906 in the transmitting terminaltransmits the observation packet at a predetermined time interval(S1101). The relay device writes into the observation packet thecongested state information (loss rate, stay time, and measurement time)measured at the reception buffer managing unit 1002. The receivingterminal measures the loss rate and the delay time of the datatransmitted from the transmitting terminal to the receiving terminal,and writes into the observation packet the measurement result (S1103).

The receiving terminal returns the observation packet having themeasurement result to the transmitting terminal (S1104). Thetransmitting terminal causes the transmission quality managing unit 906to calculate the loss rate caused by congestion out of the receivedobservation packet (S1105). The transmitting terminal controls: thetransmission rate based on the loss rate caused by the congestion; andthe error correction code based on a loss rate between the transmittingand receiving terminals (S1106). Similarly, the transmission qualitymanaging unit 906 in the transmitting terminal calculates thetransmission error rate out of the received observation packet, anddetermines the forward error correction capability of the errorcorrection code processing unit 905 based on the calculated transmissionerror rate (S1107). It is noted that the transmission error rate iscalculated by subtracting the loss rate caused by the congestion fromthe loss rate between the transmitting and receiving terminals.

FIG. 10 is a flowchart showing an operation of the relay device.

The reception buffer managing unit 1002 receives to accumulate packets.When an amount of the packet is a threshold value or more, the receptionbuffer managing unit 1002 discards the packets at random (S1201). Thepacket information rewriting unit 1003 determines whether or not thereceived packets are observation packets (S1202). In the case where eachof the received packets is an observation packet (S1202: YES), thepacket information rewriting unit 1003 obtains the congested stateinformation from the reception buffer managing unit 1002, and writes theobtained congested state information into the observation packet(S1203).

According to a distribution instruction from the routing processing unit1006, the destination distributing unit 1004 distributes the packets toan appropriate queue included in the transmission buffer managing unit1007 (S1204). The transmission buffer managing unit 1007 transmits thepacket at a predetermined time rate on a queue basis (S1205).

FIG. 11 is a flowchart showing transmission processing of theobservation packet.

The relay device determines whether or not routing between the relaydevices has been renewed (S1301). When the routing has been renewed(S1301: YES), the relay device determines that a change in a logicstructure of the network leads to a change in the degraded state of thetransmission quality, and transmits the trigger packet to thetransmitting terminal (S1303). This prompts the relay device to transmitthe observation packet to the transmitting terminal.

Further, the relay device determines whether or not the reception statebetween the relay devices has changed (S1302). When determining that thereception state between the relay devices has changed (S1302: YES), therelay device determines that a change in the connection conditionbetween the relay device and the transmitting terminal or the receivingterminal leads to a change in the degraded state of the transmissionquality, and transmits the trigger packet to the transmitting terminal(S1303). This prompts the relay device to transmit the observationpacket to the transmitting terminal.

Further, the receiving terminal determines whether or not the receptionstate at the receiving terminal has changed (S1304). When the receptionstate at the receiving terminal has changed (S1304: YES), the receivingterminal determines that a change in the connection condition betweenthe receiving terminal and the relay device leads to a change in thedegraded state of the transmission quality, and transmits the triggerpacket to the transmitting terminal (S1305). This prompts the receivingterminal to transmit the observation packet to the transmittingterminal.

In addition, the transmitting terminal determines whether or not thereception state at the transmitting terminal has changed (S1306). Whenthe reception state at the transmitting terminal has changed (S1306:YES), the transmitting terminal determines that a change in theconnection condition between the transmitting terminal and the relaydevice leads to a change in the degraded state of the transmissionquality, and transmits the observation packet towards the receivingterminal (S1307).

As described above, detected are the changes in the reception states ofthe transmitting and receiving terminal and the relay device, and thechange in the routing of the relay device. Hence, the trigger packet caninform the transmitting terminal of the changes of the receivingterminal and the relay device, in addition to the change of transmittingterminal. Thus, the transmitting terminal is capable of transmittingobservation packets at a necessary timing, instead of regularlytransmitting unnecessary observation packets.

FIGS. 12A and 12B illustrate a scene showing a specific usage of awireless ad-hoc network.

A typical example of the wireless ad-hoc network may be a use of thewireless ad-hoc network for complementarily eliminating a dead zone of aradio wave in the case where the dead zone develops depending onlocations of wireless base stations. As shown in FIG. 12A, for example,a smaller cell of a base station tends to develop the dead zone. Thusthe wireless ad-hoc network can be effective in expanding acommunication area while reducing installation costs of the basestation. Here, the cell of the base station means a range within which aradio from the base station can travel; namely, a service area.

Similarly, as shown in FIG. 12B, another possible method is to use a caras the relay device to establish inter vehicle communications in orderto expand a communication area for a local communications device at anintersection. An example of utilizing the inter vehicle communicationsis to distribute a video shoot in the vicinity of the intersection viathe local communications device, such as the Dedicated Short RangeCommunication (DSRC) and a wireless LAN, in order to expand thecommunication area for the local communications device through the intervehicle communications.

The use of the wireless ad-hoc network makes possible expanding thecommunication area even through a complex area, such as an intersection,and providing a video covering a blind area to the car. This assistssafe driving.

FIG. 13 illustrates a control technique of the observation packet andthe trigger packet based on a usage scene.

Focusing on a move of a terminal (moving car and portable terminal), anexpected high demand is to watch a received video in high image qualitywhen the moving terminal arrives at a priority monitoring area(intersection, downtown area, and alley). To meet the demand, aninterval for transmitting the observation packet and the trigger packetcan be shortened for greater resilience to congestion and a transmissionerror, so that the received video can be watched in high image quality.It is noted that installing a Global Positioning System (GPS) and adatabase of locations of priority monitoring areas in the terminalreadily realizes detection of the terminal arriving at the prioritymonitoring area.

Another expected high demand to watch a received video in high imagequality is the case where a moving terminal going straight ahead changesthe moving direction. An exemplified case is that the moving terminal isa car, and the car is assumed to give a turn signal. To meet the demand,an interval for transmitting the observation packet and the triggerpacket can be shortened for greater resilience against congestion and atransmission error, so that the received video can be watched in highimage quality. It is noted that detection of a change of a movingdirection of the moving terminal can be achieved by sequentiallyobtaining: a condition change of a turn signal when the moving terminalis a car; and GPS-based position information when the moving terminal isa portable terminal, and by detecting a temporal change of each of thecondition change and the position information.

Further considered is a case where a user has more opportunities towatch a video in high quality when the moving terminal makes atransition from a high speed moving state to a low speed moving state orto a complete stop state. To meet the demand, an interval fortransmitting the observation packet and the trigger packet can beshortened for greater resilience against congestion and a transmissionerror, so that the received video can be watched in high image quality.It is noted that the detection of the moving speed of the terminal canbe achieved by sequentially calculating position information on the GPSand by detecting a temporal change of the position information.

Further, detection of a change of a transmission condition of the movingterminal expects a high demand to watch a received video in high imagequality. Here, the change of the transmission condition specificallymeans at least one of the changes of the loss rate, the reception fieldintensity, and the transmission speed. To meet the demand, an intervalfor transmitting the observation packet and the trigger packet can beshortened for greater resilience against congestion and a transmissionerror.

By detecting the change of the transmission condition of the movingterminal, for example, the facts that the moving terminal has entered alocal communications area, or the moving terminal has performed handoverto another base station are detected. Accordingly, a short interval isset for transmitting the observation packet and the trigger packet for acertain period in establishing a new communication link. This allows astable transmission condition to be quickly established.

When utilizing the inter vehicle communications, moreover, cars making astop before the intersection slow down the moving speed of the terminal.This provides stable communications (inter vehicle communications)between terminals, and improves the transmission quality. Inestablishing a transmission path between terminals when the terminalslows down the moving speed, as described above, a short interval is setfor transmitting the observation packet and the trigger packet for acertain period in establishing the transmission path. This allows astable transmission condition to be quickly established. It is notedthat the above may be applied to a portable terminal including acellular phone, as well as a car.

As described above, the present invention involves transmitting anobservation packet from a transmitting terminal towards a receivingterminal via the relay device. Upon receiving the observation packet,the relay device writes into the observation packet congested stateinformation indicating a data loss rate at the relay device, and relaysthe observation packet. Further, the receiving terminal writes into theobservation packet degraded state information indicating a data lossrate between the transmitting terminal and the receiving terminal, andreturns the observation packet to the transmitting terminal. Hence, thetransmitting terminal can specify a congested relay device out of thecongested state information written in the observation packet. Inaddition, subtracting the congested state information from the degradedstate information makes possible calculating a transmission error ratein the wireless ad-hoc network. Assigning, to data to be transmitted, aforward error correction code suitable for the transmission error ratecan realize high quality transmission.

Moreover, the embodiment allows the transmitting terminal to transmitthe observation packet when a change occurs in a reception state of thedata. Hence, the present invention makes possible preventing thetransmitting terminal from unnecessarily transmitting the observationpacket, and alleviating loads provided to the network and to theterminal and the relay device.

Further, when receiving plural instructions (trigger packets) fortransmitting an observation packet to be transmitted from the samesource to the same destination in a short period of time, the relaydevice keeps one of the trigger packets and discards the rest. This canprevent two or more trigger packets having the same details from beingtransmitted towards the transmitting terminal. This can alleviate loadsdelivered to the network, the terminal, and the relay device.

Between the transmitting and receiving terminals connected to the relaydevice, moreover, the present invention selects intensity and atechnique of a forward error correction based on a transmission error,and controls a transmission rate comprehending a degree of an effect ofeach of congestion and the transmission error. This allows the presentinvention to realize high quality video transmission.

Although only an exemplary embodiment of this invention have beendescribed in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiment without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention.

INDUSTRIAL APPLICABILITY

A relay transmission technique in the present invention realizestransmission of videos and speeches in high quality with a use of anad-hoc network, and is applicable to an indoor monitoring system, astreet monitoring system and the Intelligent Transport Systems (ITS). Inaddition, the relay transmission technique, applicable to transmittingvarious kinds of media including text, still pictures, and music, inaddition to videos and speeches, can perform indoor as well as outdoormedia transmission in high quality.

1. A data transmitting and receiving system comprising: a relay devicewhich is connected to a network; and a first terminal and a secondterminal each of which transmits and receives data via said relaydevice, wherein said relay device includes: a receiving unit configuredto receive an observation packet used for observing transmissionquality, and data; a reception buffer in which the data is stored; areception buffer managing unit configured to observe a loss rate of thedata in said reception buffer; a packet information rewriting unitconfigured to write the loss rate into the observation packet ascongested state information, the loss rate being observed by saidreception buffer managing unit; a transmission buffer which stores on adestination basis the data stored in said reception buffer; atransmission buffer managing unit configured to forward the data storedin said transmission buffer at a predetermined transmission rate on thedestination basis; and a transmitting unit configured to transmittowards a destination the data forwarded by said transmission buffermanaging unit, and the observation packet, said first terminal includes:a first transmitting and receiving unit configured to transmit andreceive data and the observation packet; and a first transmissionquality managing unit configured to write degraded state information inthe observation packet, the degraded state information indicating theloss rate of the data between said first terminal and said secondterminal, and said second terminal includes: a second transmitting andreceiving unit configured to transmit and receive data and theobservation packet; a second transmission quality managing unitconfigured to calculate a transmission error rate of the data in saiddata transmitting and receiving system by subtracting the loss rateindicated in the congested state information from the loss rateindicated in the degraded state, the loss rates being included in theobservation packet obtained from said first terminal; and an errorcorrection processing unit configured to execute processing on data tobe transmitted, so that an error of the data is corrected when the datais received, the processing being executed according to the transmissionerror rate calculated by said second transmission quality managing unit.2. The data transmitting and receiving system according to claim 1,wherein said error correction processing unit is configured to determineforward error correction capability according to the transmission errorrate calculated by said second transmission quality managing unit so asto assign, to the data to be transmitted, a forward error correctioncode based on the forward error correction capability.
 3. The datatransmitting and receiving system according to claim 1, wherein saidfirst terminal further includes a first terminal reception statemanaging unit configured to detect a change of a reception state of thedata in said first transmitting and receiving unit, and, in the casewhere the change of the reception state is determined, to instruct saidfirst transmission quality managing unit to transmit the observationpacket, said second terminal further includes a second terminalreception state managing unit configured to detect a change of areception state of the data in said second transmitting and receivingunit, and, in the case where the change of the reception state isdetermined, to instruct said second transmission quality managing unitto transmit the observation packet, said first transmission qualitymanaging unit is configured to transmit, via said first transmitting andreceiving unit, the observation packet towards a terminal of adestination of the data in response to the instruction from said firstterminal reception state managing unit so as to transmit the observationpacket, and said second transmission quality managing unit is configuredto transmit, via said second transmitting and receiving unit, theobservation packet towards a terminal of a destination of the data inresponse to the instruction from said second terminal reception statemanaging unit so as to transmit the observation packet.
 4. The datatransmitting and receiving system according to claim 3, wherein each ofthe reception states of the data includes at least one of receptionfield intensity, detection of notification information of handover, atransmission speed, a selected forward error correction technique orselected intensity, a transmission error rate of the data, and thenumber of retransmissions of the data.
 5. The data transmitting andreceiving system according to claim 3, wherein said first terminalreception state managing unit is configured to detect a change of theloss rate of the data indicated in the congested state information orthe degraded state information included in the observation packet, and,in the case where the change of the loss rate of the data is detected,to instruct said first transmission quality managing unit so as totransmit the observation packet, and said second terminal receptionstate managing unit is configured to detect the change of the loss rateof the data indicated in the congested state information or the degradedstate information included in the observation packet, and, in the casewhere the change of the loss rate of the data is detected, to instructsaid second transmission quality managing unit so as to transmit theobservation packet.
 6. The data transmitting and receiving systemaccording to claim 1, wherein said relay device further includes a relaydevice reception state managing unit configured to detect a change of areception state of the data in said receiving unit, and, in the casewhere the change of the reception state is determined, to instruct saidsecond transmission quality managing unit so as to transmit theobservation packet, and said second transmission quality managing unitis configured to transmit, via said second transmitting and receivingunit, the observation packet towards said first terminal in response tothe instruction from said relay device reception state managing unit soas to transmit the observation packet.
 7. The data transmitting andreceiving system according to claim 6, wherein said relay device furtherincludes: a routing processing unit configured to generate a routingtable indicating a transmission path of the data, using a predeterminedrouting protocol; and a destination distributing unit configured todetermine a next destination of the data according to the routing table,said transmission buffer managing unit is configured to transmit thedata towards the destination determined by said destination distributingunit via said transmitting unit, and said relay device reception statemanaging unit is configured to detect renewal of the routing table bysaid routing processing unit, and, in the case where the renewal of therouting table is determined, to instruct said second transmissionquality managing unit so as to transmit the observation packet.
 8. Thedata transmitting and receiving system according to claim 6, whereinsaid relay device reception state managing unit is configured to detectthe change of the loss rate of the data indicated in the congested stateinformation included in the observation packet, and, in the case wherethe change of the loss rate of the data is determined, to instruct saidsecond transmission quality managing unit so as to transmit theobservation packet.
 9. The data transmitting and receiving systemaccording to claim 6, wherein, in the case where said receiving unitreceives within a predetermined time period a plurality of instructionsincluding the instruction for transmitting the observation packet from asame source to a same destination, said relay device reception statemanaging unit is configured to execute one of the instructions on saidsecond transmission quality managing unit, and to discard remaininginstructions out of the instructions.
 10. A terminal, connected to arelay device via a network, which transmits and receives data to andfrom another terminal via the relay device, said terminal comprising: atransmitting and receiving unit configured to transmit and receive dataand an observation packet used for observing transmission quality; and atransmission quality managing unit configured to write degraded stateinformation in the observation packet, the degraded state informationindicating a loss rate of the data between said terminal and the otherterminal.
 11. A terminal, connected to a relay device via a network,which transmits and receives data to and from another terminal via therelay device, said terminal comprising: a transmitting and receivingunit configured to transmit and receive an observation packet used forobserving transmission quality, and data; a transmission qualitymanaging unit configured to calculate transmission error rates of thedata in said terminal and in the relay device by subtracting a loss rateof data, in a reception buffer in the relay device, which is indicatedin congested state information from a loss rate of the data, betweensaid terminal and the other terminal, which is indicated in degradedstate information, the degraded state information and the congestedstate information being included in the observation packet obtained fromthe other terminal; and an error correction processing unit configuredto execute processing on data to be transmitted, so that an error of thedata is corrected when the data is received, the processing beingexecuted according to the transmission error rate calculated by saidtransmission quality managing unit.
 12. A relay device, connected to aterminal via a network, which relays data transmitted to and receivedfrom terminals including the terminal, said relay device comprising: areceiving unit configured to receive an observation packet used forobserving transmission quality, and data; a reception buffer in whichthe data is stored; a reception buffer managing unit configured toobserve a loss rate of the data in said reception buffer; a packetinformation rewriting unit configured to write the loss rate into theobservation packet as congested state information, the loss rate beingobserved by said reception buffer managing unit; a transmission bufferwhich stores the data stored in said reception buffer on a destinationbasis; a transmission buffer managing unit configured to forward thedata stored in said transmission buffer at a predetermined transmissionrate on a destination basis; and a transmitting unit configured totransmit towards a destination the data forwarded by said transmissionbuffer managing unit, and the observation packet.
 13. A datatransmission method for a system which includes a relay device, atransmitting terminal, and a receiving terminal that are connected to anetwork, and transmits data from the transmitting terminal to thereceiving terminal via the relay device, said data transmission methodcomprising: transmitting data from the transmitting terminal to therelay device towards the receiving terminal; relaying the data by therelay device; receiving the data by the receiving unit; transmitting anobservation packet used for observing transmission quality from thetransmitting terminal to the relay device towards the receivingterminal; receiving the observation packet by the relay device, writinga loss rate of the data in the observation packet as congested stateinformation, and transmitting the observation packet towards thereceiving terminal; receiving the observation packet transmitted fromthe relay device by the receiving terminal, writing a loss rate of thedata into the observation packet as degraded state information, the dataregarding between the transmitting terminal and the receiving terminal,and transmitting the observation packet towards the transmittingterminal; and calculating a transmission error rate of the data in thesystem by the transmitting terminal receiving the observation packethaving the congested state information and the degraded stateinformation, and subtracting the congested state information from thedegraded state information included in the received observation packet.14. A terminal, connected to a relay device via a network, whichtransmits and receives data to and from another terminal via the relaydevice, said terminal comprising: a receiving unit configured to receivean observation packet used for observing transmission quality, and data;a transmission quality managing unit configured to calculate atransmission error rate of the data in said terminal and the relaydevice according to a loss rate indicated in degraded state informationand a loss rate indicated in congested state information on the datareceived by said receiving unit, the degraded state information and thecongested state information being included in the observation packetobtained from the other observation packet; and an error correctionprocessing unit configured to execute error correction processing ondata to be transmitted according to the transmission error ratecalculated by said transmission quality managing unit.